Bench pressing is by far the most popular lift when it comes to fitness, I can't tell you how many times someone has noticed my physical appearance and asked me "how much ya bench." Now I like benching just as much as the next guy but c'mon guys there are other lifts.
When it comes to benching, arching is the most controversial subject. Keep in mind this is completely subjective.
When I first started lifting, I was your typical gym rat where every day was bench day. I would bench 5 days a week, if not, 6 or 7 days a week with, of course, a flat back. After continuously lifting this way, I found my shoulders getting tighter and my nerves pinched. It wasn't until I started arching my back that I found that I could bench 6 days a week with little to no stress placed on my shoulders.
Now everyone can agree that keeping your chest up during the movement is beneficial, and if that's the case then arching would be the most optimal way to perform the movement. I can say that I have been lifting with an arch for over 4 years now and have not experienced any pain in or around my back or shoulders. You can argue that arching your back causes back problems down the road, but I've known several powerlifters who have been benching with an arch their entire life and have not had a single back problem. I'm not saying that arching is completely safe, there are risks to everything. Arching is relative to each individual and each individual has to work on their arch to suit their level of flexibility. For my body type and my level of flexibility arching is the best bet to keep me safe and to keep me lifting for more years to come.